There is no all-over ability Red Frag, as each group (Awakened, Coder, Hacker, Operative) has its own Red Frag at each level. Meaning an Awakened X Red Frag would go to an Awakened ability and so on for Hacker, Coder, and Operative abilities.

First, make sure you are killing the right mobs for the Red Frags. I've killed the wrong gang many times in International before I started to memorize them from use.

Next, you need to understand that farming Red Frags is slow going. Expect maybe 4-10 Frags/hour depending on the gang and how fast you can kill them.

If you are killing the right ones, you'll want to kill bosses and the highest level/rank gang members you can find in the district for most frequent drops. Still, if you can kill them all in two or three shots, don't run around the district just killing Rank 3s as you will spend more time running around then actually killing gang members.

It appears that the Red Frag drops are distributed across all Fake Agent (LED or N30) spawns around the same level. If the gang you need has been replaced by Fake Agents, your best bet is to still farm at the old area, but your Red Frag drops will be slightly more infrequent and random.

In summary, your time will be tougher if your mob has been replaced by Fake Agents, but it is still doable.